Long-sought “glueball” particle may be found, physicist says
Special to World Science
Physicists have been on a three decades’ long search for a
strange type of subatomic particle called a glueball.
But the hunt may be almost, or already, over, a researcher
claims. And if that’s true, it could clarify what nature’s most
fundamental particles are.
The glueball quest is connected with a popular
theory called quantum chromodynamics, which claims matter’s most basic
components are tiny entities called quarks. Other particles, called “gluons,”
act as a “glue” that binds quarks together to form the protons and neutrons of the atomic nucleus.
|According to the theory of quantum
chromodynamics, the known components of the atomic nucleus, called protons
and neutrons, are actually composed of triplets of "quarks,"
shown in red above. Particles known as gluons carry a force that holds the
quarks together so that they are practically inseparable. (Courtesy U.S.
Department of Energy)
particle physicists consider the theory definitive; atom-smashing experiments
have confirmed it, says Michael Chanowitz, a theoretical physicist at Lawrence Berkeley National Laboratory in Berkeley,
Yet one of
its most dramatic predictions, he added, has yet to be verified. That is the
existence of glueballs, particles made only of gluons.
would be “an intriguing new form of matter,” he said. Little is known about
what they’re like, and what they might be useful for—probably nothing, he
added. But their discovery could raise new questions that lead to further
progress in physics, and as for their practical applications, “you never know.”
way, he said, glueballs would certainly be unique, because they
would be the only force-carrying particles known to stick together.
is in contrast to particles such as photons, which we see as light. Photons
carry the electromagnetic force, which makes electrically charged objects
attract or repel each other. But photons themselves don’t interact, as they
have no charge. Gluons attract each other because do have a sort of charge,
whimsically called “color charge” though it has nothing to do with color.
Beyond this basic description, the properties of
glueballs are murky. That largely explains why physicists haven’t been able to
find them, Chanowitz said: researchers don’t quite know what to look
But, he argued, a solution may be at hand.
It’s believed that newly
would quickly decay, or fall apart, producing other particles in the
process. Chanowitz says a glueball could
be identified by the types of particles it decays into. He detailed his proposal in the October 21, 2005 issue of the research journal
Physical Review Letters.
Traditionally, physicists “thought that glueballs decay equally [often] to pairs of three different types of quarks,”
Science@BerkeleyLab magazine, a publication of the laboratory.
But his calculations, he added, shows that when the glueball undergoes one
common type of decay—into pairs of particles—those tend to consist of a
particular type of quark, called the strange quark.
“We can use this signature to hunt” for glueballs, he told the publication, adding that he relied on a technique called all-orders perturbation theory to reach his
His said his findings fit with puzzling results from supercomputer calculations done ten years ago by Donald Weingarten and colleagues at the IBM Watson Research Center in Yorktown Heights, N.Y.
Contrary to previous wisdom, they found that the lightest glueball decays more often to pairs of so-called K mesons, particles partly composed of strange quarks, than to much lighter “pi mesons.” These are composed of other types of quarks called “up” and “down.”
Chanowitz said Weingarten’s results are due to the lightest glueball’s preference to decay to strange quark pairs.
“My work adds credence to Weingarten’s seemingly counterintuitive results and provides an explanation for what was then an unexpected finding,” Chanowitz told
Science@BerkeleyLab. “It shows that we’re on track.”
Chanowitz’s work also tightens the focus on a particle that physicists have eyed as the lightest glueball, also called the scalar glueball, for several years, the publication reported. The particle, f(1710), possesses many characteristics of a glueball, but it has been dismissed because it predominantly decays to K-meson pairs.
“But my analysis implies this is exactly what it should do,” Chanowitz told the
magazine. He added that studies with supercomputers and particle-smashers could clear up the question in a few years.
The glueball quest might lead to further
interesting findings about the nature of matter, Chanowitz told World
Science, because glueballs are linked to poorly understood aspects of
quantum chromodynamics that in turn affect the properties of other
No one knows
what else the hunt could lead to, he said—possibly nothing. But then again, he
added, the physicist Ernest Rutherford famously insisted his discovery of the
atomic nucleus would have no practical applications, not long before nuclear
bombs and nuclear energy made their appearance.
“From the point of view of the basic research scientist,
one of our most valuable products is the next question that our research allows us to ask,”
he remarked. “And we never know what the question is going to be until we’ve completed what we’re working on now.”
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